1. Field of the Invention
The present invention relates to a laser desorption ion source, and more particularly, to a laser optical bench for use with a laser desorption ion source that preferentially shapes a beam from a light source by predominantly focusing the beam in a single plane.
2. Description of the Prior Art
A laser desorption ion source is a device that utilizes the energy inherent in a focused laser beam to promote the desorption of neutrals and/or ions from solid or liquid state matter. In the case of solid matter, materials or samples of interest are presented as solid state crystals or thin films upon a sample support typically referred to as a probe. For liquid matter, the fluids are introduced as droplets or a fine spray and may be desorbed in stream or upon a physical support.
The energy transfer process may proceed through direct thermal or electronic excitation of the material or through indirect thermal excitation. If the material directly absorbs energy from the laser source and heats up via direct thermal or secondary thermal changes in response to electronic excitation, the process is known as laser-induced thermal desorption (LITD). If the material of interest receives thermal energy from neighboring compounds while being a member of a co-crystal or thin film matrix, the process is known as matrix-assisted laser desorption (MALD). If the material or sample of interest has been physically modified, extracted or amplified by the probe surface, or if the probe surface contains integral energy absorbing molecules capable of indirect energy transfer to the sample of interest, the process is known as surfaced enhanced laser desorption (SELD).
Should preferential ionization be created for the above described desorption motifs, then such processes are respectively referred to as laser desorption/ ionization (LDI), matrix-assisted laser desorption/ionization (MALDI), and surface enhanced laser desorption/ionization (SELDI).
Regardless of which energy transfer process is used, a laser desorption ion source primarily consists of a collection of components generally referred to as a laser optical bench. Such a laser optical bench is schematically represented in FIG. 1.
Generally, a laser optical bench 10 includes a light source or photon source 11, which is generally a continuous beam or pulsed laser, a beam splitter 12, photodiode or other photodetector 13, attenuator 14, lens 15, mirror 16 and target 17, which is generally a probe including a sample of material of interest.
If a continuous beam laser is employed as light source 11, desorption/ionization occurs with a constant duty cycle. If desired, high speed gating of the beam is typically achieved by using a shutter, which blocks the beam or a movable mirror that directs the beam into a beam dump (not shown). If a pulsed laser is employed as light source 11, the duty cycle is dependent upon the pulse width and repetition rate. High speed gating of the beam is achieved by controlling the pulsing process.
In some situations, the laser optical bench may include a photodetector or photodiode 13 to measure the energy of the laser source or to detect the lasing event in the case of pulsed laser applications. Typically, optical beam splitter 12 is used to divide off a small fraction of the incident beam and direct it toward the appropriate photodetector. If the photodetector is used to measure delivered energy, it is usually of the thermal, photo-emissive, or semiconductor detector varieties. If the photodetector functions to detect the lasing event of a pulsed laser train, the photodetector is preferentially a small surface area semiconductor photodiode, which is capable of delivering very fast response times.
The propagated laser beam needs to be processed for the purposes of laser desorption. Such processing often involves control of laser energy, laser fluence (laser energy/unit area), and/or laser irradiance (radiant power/unit area). To achieve the latter, a combination of lenses and attenuation devices are often used. Typical laser energy attenuation devices include a mechanical iris, a neutral density filter or a fresnel reflection/refraction device. If a neutral density filter has a gradient of optical densities allowing for continuous adjustment of transmitted laser energy, it is referred to as a gradient neutral density filter (GNDF).
The ultimate size of the focused laser spot on the target is controlled through prudent selection of mirrors and lenses. Typically, a design that optimizes optical throughput while providing the desired fluence or irradiance dynamic range is employed. Additionally, the combination of attenuating and focusing elements should optimally create an image whose spatial distribution creates a desorption locus that promotes maximum sampling area while maintaining maximum ion extraction efficiency.
Increasing sampling area has three major advantages, specifically decreased analysis time, improved sample-to-sample reproducibility, and increased analytical sensitivity. The advantage of decreased analysis time is readily apparent and generally desirable. If one addresses a greater amount of sample area with each laser spot, a given sample region may be completely interrogated in less time than that required by approaches that employ smaller laser spots.
Typical sample preparation techniques for the previously noted laser desorption scheme inherently create solid-state or liquid samples with appreciable amounts of heterogeneity and microenvironmental differences. These differences are sources of qualitative and quantitative in reproducibility when assaying a plurality of identical samples. Although some approaches, such as SELDI, function to minimize these effects, statistically significant perturbations may still be observed. The employment of large laser probed regions improves reproducibility by increasing the area of sample investigated for each laser desorption event, statistically minimizing the effect of microheterogeneity.
The means by which target probed areas are enlarged is important with respect to sample laser irradiance. Generally speaking, sample desorption and ionization for the previously identified schemes occur at some threshold irradiance level. Furthermore, it is often desirable to have the ability to operate at levels significantly higher than threshold. Consequently, a given increase in laser spot area would require a concomitant increase in laser radiant power. Such laser radiant power increases may result in the need to employ more powerful and expensive laser sources. Accordingly, so, a means which increases the target sampling area that does not necessitate significant increases in laser radiant power is desired.
Increased analytical sensitivity is achieved by virtue of the fact that more sample is desorbed and ionized for each desorption event, assuming that the additional ionized material within the desorption cloud may be efficiently extracted. The desorption cloud can be considered to be a collection of ions, neutrals, and electrons capable of shielding externally applied electrical fields. It is generally recognized that ion extraction occurs within a given axial length of the desorption cloud known as the plasma skin depth. The plasma skin depth is that portion of a cloud""s outer perimeter for which externally applied electric fields penetrate and do work upon charged particles. It is typically determined by the fundamental energetics of the desorption process and for a given set of conditions, is considered to be relatively dependent upon the cloud""s charged particle density.
For the previously noted techniques, desorption cloud charge particle density has been determined to be dependent upon applied laser irradiance. Low irradiance levels produce clouds of nominal charged particle density. Under these conditions, the plasma skin depth can extend appreciably into the center of the desorption cloud and a vast majority of the desorbed ions can be efficiently extracted. In contrast, the application of high irradiance levels create clouds of extreme charged particle density, producing a plasma skin depth that is a fraction of the total cloud size, thus providing for sub-optimal levels of ion extraction. The distinction of low versus high laser irradiance levels is dependent upon the ionization technique. For the applications of SELDI and MALDI, high laser irradiance can be considered to be that which exceeds 10 mW/cm2.
From the previous explanation, it becomes clear that optimum ion extraction efficiency will be achieved under conditions for which a maximum population of desorbed ions reside within the plasma skin depth. In this manner, laser spot geometries that promote desorption clouds with maximized surface area to volume ratios are favored.
Further complicating this process is the requirement for creating homogeneous energy, fluence, or irradiance profiles across the laser spot. During the process of desorption and ionization, the initial energy conditions of these gaseous products have been shown to be somewhat dependent upon the initial amount of applied laser energy. If the laser image contains positional dependent energy gradients or hot regions, desorbed products from different regions may exhibit significantly different initial energies. This condition may be detrimental to mass analysis, especially if non-orthogonal time-of-flight mass spectrometric techniques are employed.
A laser optical bench, in accordance with the present invention, for use with a laser desorption/ionization mass spectrometer addresses the shortcomings of the prior art. Such a laser optical bench includes a laser for producing light, a beam expanding focusing structure that receives light from the laser and focuses it in predominantly a single plane, an attenuator that receives light from the beam expanding focusing structure, a beam steering structure for directing light from the attenuator to a target, and an omnidirectional focusing element for focusing light from the beam steering structure on the target.
The combined action of the aforementioned elements generally serves the purpose of minimizing laser spot energy heterogeneity while creating a target probe sampling spot geometry of enlarged surface area and a desorption cloud with maximized surface area to volume ratio.
In accordance with further preferred aspects of the present invention, the beam expanding focusing structure consists of a pair of cylindrical lenses, and the laser optical bench further includes a plano convex lens that focuses the light from the beam steering structure onto the target probe.
In accordance with another preferred aspect of the present invention, the first cylindrical lens of the beam expanding focusing structure preferentially focuses the laser beam in a single plane with respect to a gradient neutral density filter attenuator. The orientation of the focusing plane is aligned with the gradient direction of the neutral density filter so that a minimum energy gradient exists across the beam transmitted through the filter. Furthermore, because the incident beam is allowed to diverge in regions outside of the focusing plane, the laser spot area incident to the GNDF is sufficiently large so as to limit the incident irradiance to levels below that of the GNDF damage threshold. A second cylindrical lens is used to collect the transmitted beam and, in combination with the inherent beam divergence of the laser source, expand it to match the numerical aperture of the remaining optical elements.
In accordance with another preferred aspect of the present invention, the beam steering structure generally includes a mirror that reflects light to a dichroic filter. The dichroic filter allowing some light to pass therethrough while reflecting a majority of the light to the target probe. The light transmitted through the dichroic filter is then preferably passed to a piano convex lens that focuses the light onto a photodetector in order to measure the amount of applied laser energy.
Thus, the present invention provides a laser optical bench for use with a laser desorption/ionization mass spectrometer that allows for beam shaping, which is created by preferentially focusing the laser beam to a minimum dispersion in only one plane. By initially focusing the laser beam in a single plane, a decreased spatial laser energy gradient across the beam after it passes through the attenuator is realized. Furthermore, beam expansion is realized by the combined action of the second cylindrical lens and the inherent beam divergence of the laser source, thus utilizing the full numerical aperture of the system while selectively allowing expansion in only one dimension. Finally, ion desorption loci are created that are shaped in a manner that optimizes ion collection/extraction efficiency.